Wave-suppression circuit



March 10, 1931. J FARRINGTQN I 1,795,484

WAVE SUPPRESS ION G IRCUIT Original Filed July 31, 1923 A TTOHWEY Patented Mar. 10, 1931 UNITED JOHN F. EARRING-TON, OFFLUSHING', NEE/V YOEK, ASSZ-IGNGE TO WESTERN ELECTRIC I COLIPANY, INCORPORATED, OF Ii F-Vl YGEEZ, N. Y., A CORPORATION OF NEW'YORK" Original application filed July 81, 1 '2 Serial "Jo Divided t-" This invention relates to wave suppression circuits, particularly in the form of wave modulating systems employing electric space discharge devices as modulators.

The invention provides a modulating device in which all unmodulated-waves of a range of frequencies impressed on the device can be suppressed to-any'desired degree, without necessitating employment of a plurality of discharge paths in the device.

This application is a division of applica tion Serial No. 654,823, fil d July 81, 1928, which eventuated into Patent 1,724,917. granted August 20, 1929.

In the specific embodiments of the inven tion shown in the drawings by way of example, the modulating device is a three-electrode electric space dischar e tube, with a resistance connecting its control electrode or grid to its anode, the resistance having such magnitude as to transmit to the cathodeanode circuit a current adequate to balance out, or neutralize, or counteract to any desired degree, all currents of the impressed wave frequencies which are transmitted through the device. Thus the waves im pressed on the modulator input circuit can be excluded from the modulator output circuit, and the waves in the otuputcircuit restricted to modulated waves or products of the modulating or distorting action of the device due to the curvature of its input voltage-output current characteristic.

Fig. 1 is circuit diagram for facilitating explanation of the principleot' operation of a single tube carrier suppression modu lating system;

Figs. 1A and 1B indicate specific arrange ments of impedance elements which may be employed in the circuit, Fig. 1;

Fig. 2 represents an arrangement whereby the completeness of carrier suppression may be estimated and the circuit accurately adjusted to increase the completeness of the carrier suppression;

Fig. 3 shows a simple transmission system employing a single tube carrier suppression modulator; and

Fig. 4 is a circuit diagram illustrating the utilization of-a circuit, such. as that shown SIGN OEECUFLT Patent Ho. 1,724,917, dated August 20, 1929.

Serial No. 198,444.

in Fig. 3, in a partial ca rier suppression d1 crence frequency duplex telephone sys-- L rein.

In Fig. 1, the input circuit of a modulating I tube M-4O is supplied with carrier waves from a source 41 and signaling waves from a source 42. the tube M-40 is connected to a receiving in- The plate-filament circuit of strument 43 through a filter 44. An impedance network 45 is connected between the ,j

grid and the plate of the tube M40. The usual batteries and the choke coil 46, together with stopping condenser 77 arewemployed in the circuit as indicated. in a circuit such as illustrated in Fig. 1, 1t 1s POSSlblQ'lZO sup-. press from the output circuit current of the carrier frequency supplied by source 41and current or" the signaling frequency supplied 5 by source 42. The physical principles on.

that impressed on its grid but opposite in phase; or, from another point of v1ew,asa means for transmitting an impressed elec tromotive force wave through a circuit of 1m 'pedan'ce R (the plate-filament impedance)- with an amplification and a reversal of phase. It is a function ofthe network, con-' nected between the grid and plate, to trans-- mitto the same points in the output circuit an electromotive force equal and opposite to the electromotive force from the virtual source. In order to transmit through the network 45 an electromotive force which will be equal to that transmitted through the device, the impedance of the network must satisfy the relation Since impedance R is substantially a pure resistance, the impedance Z must be effectively a pure resistance, as it'is in the Nichols systeniu In this system. the network coma prises a pure resistance and a plurality of re active elements which are tuned to the frequency atwhich the balance is to be ell'ected. The present invention constitutes an improved Nichols arrangement to the extent of balancing out all the electromotive force waves impressed on the grid-filament circuit, by providing a networl-z whose impedance is substantially a pure resistance for all of these frequencies. It therefore provides a system in which a single tube can be used to accomplish substantially the same results as could otherwise be accomplished by the use of a push-pull arrangement of two tubes as illustrated, for example, by U. S. patent to Hartley, 1,419,562, June 13, 1922.

The above relation holds only for frequencies applied to the input circuit and not to those frequencies which result from the curvature of the tube characteristic, and which appear in the output circuit. Among the components of the output current are the sum and difference frequencies resulting from intermodulation of the waves impresscifl on the input circuit and also harmonics of the impressed waves. lVith the value of impedance 415 corresponding to the above mentioned condition, if the v ltage amplitude of waves of carrier frequency 7 from source 41 impressed on the grid circuit is represented by 6 and if the source 42 is a source of signaling frequency 3 which applies to the tube M-l0 a voltage of amplitude 0 the output voltage of frequency 2; will be 2 and of 9 A 2622 M T and of the frequencies f+s and f-s, i l 1. 0 6 where A is a constant depending on the tube. In order that the amplitude of the output voltage of frequency 28 (or other multiples of the signaling frequency) may be small as compared with the amplitude of the output voltages of frequencies f+s or s, the value of 6 should be large as compared to 0 The double frequency 2f will lie outside the range of fs or f+s and can be suppressed by the filter circuit Inasmuch as the impedance 45 should satisfy the equation for the frequencies f and s, it may approximate a pure resistance.

Fig. 1a discloses one arrangement in which a. large condenser a? and a variable resistance are connected between the grid and the plate of the tube MO. The fun tion of the condenser l7 is not related to that of the resistance e8 ano is necessary only to insure, when the combination is used in a practical vacuum tube circuit, that the plate-filament potential is not communicated to the grid. If

capacity is of large enough value, this result may be secured without sensibly affecting the value or character of the impedance of the combination as a w..ole. In Fig. 1/), the variable lance 4:8 is placed in series with a source of clectromotivc force 49 so adjusted as to balance the plate-filament battery of the tube 11- 10. The source ll) has its positive terminal connected to the plate.

The sul'ipressing effect of such an arrangement as illustrated in Fig. 1 may be verified experimentally for audible frequencies by an arrangement such as that of Fig. 2. A source 50 supplies waves of any frequency for which the circuit is to be tested. lhis current is supplied to the tube input circuit through the transformer as shown and the telephone receiver 4-3 is connected to the output circuit. The receiver 51 is directly connected to the source 50. By listening at receivers 4-3 and 51 simultaneously and adjusting the resistance 48, the nirticular value of the resistance may be found for which the only tone heard in the receiver 23 corresponds to a harmonic or mul ole frequency of the tone heard in receiver o1. ii hen speech currents are supplied from source 50, the condition for balance is not so exactly defined with a single tone beca. so the double frequency of S3CCl1 in general constitutes intelligible sounds. For speech frequencies, therefore, the actual adjustnicnt is made at that point which the operator concludes that the sound produced by receiver 4:8 consists principally of harmonic frequencies.

A. simple transmission circuit involving a single tube carrier suppression modulator is shown in Fig. 3. Source 41 supplies waves f carrier frequency and source 4-2 waves of signaling frequency to the modulating tube ill-41G. lhese elements are shown as serially included in the input circuit, which should be taken to indicate diagran'n atically any known method of supplying waves of these frequencies to the input circuit by means of inductive, capacitative, or resistance couplings arranged in series or parallel. The network consists of the large condenser l7 and adjustable resistance As mentioned above, the amplitude of the desired modulated components may be made large as compared to the amplitude of the harmonics of the audible signaling frequencies by making the carrier wave voltage large with respect to the signaling wave voltage. This results in a relatively large amplitude harmonic of the carrier wave frequency, which may be suppressed by the low pass or band pass filter M, which suppresses multiple frequencies of the car ier wave source and may also suppress multiple frequencies of the signal wave source. If desired, the filter 44- may have its cut-oil limit at such a point as to supessee pre'ss one side band of the modulated e've.

Tlie" cut-'oif fnay also beisucli' thattlie filter accomplishes additional up resses of the" carrier" wave. The currents transferred through the filter 44 areamplified suitable amplifying system, such as the amplifier A '52,- andrad1ated from antenna 53 oro'th'erwise transmitted or utilised as maybe just'ed so that the' unmodiilated ccinponnt of carrier frequency current isno t entirely suppressed for a reason whichwill' be'i here-inafter stated. p

In Fig. 4 the source 60 suppliesfwa ves of some convenient base frequency, for example 50,000 cycles per second. A harmonic generator H'6I is connected to the source 60 and produces waves of a suitablemultipl'e' fre quency,'forexample,' 500,000"cycles per second, which are selected by any suitable selec tive network 62. A proper voltage of this frequency may be taken off from the potentiometer 63 and applied to the input circuit of the modulator Ml0. Signaling Waves to I be transmitted are supplied from the microi phone circuit 64 to the input circuit of the rier frequency and the network serves to tune the antenna for the incoming carrier frequency. If desired, a circuit connection 68 may be employed to impress upon the input circuit of the amplifier AD6 an adjustable small component of the outgoing carrier wave frequency. For this purpose the connection 68 is provided with an adjustable tap upon the potentiometer 63. There are thus radiated from the antenna 7 modulated carrier waves of the frequencies f s Where f represents 500,000 cycles and 8 the signal to be transmitted. A component of the unmodulated carrier frequency may be included in the radiated energy and may be regulated to any desired amount. Let it be assumed that incoming carrier waves of similar nature having the frequencies of f is where f represents 550,000 cycles, are being received. The incoming waves combine in the plate-filament circuit of the amplifier AD6 with the similar outgoing carrier Waves and owing to the detecting action of the tube, differ ence frequency waves result. These ditference frequency waves comprise in general side bands of fr q cies (firffi or frequency.

is.- or either" ofthemdepending" on which station is transmitting atthe instant 615a combination of them if both stations-fare attempting to transmit 'simul taiie ou'sly; these side'bands being based upon an intermediate carrier wave of frequency 72'- f which however is not/present as such. These waves are selected from other interfering waves 'by'a suitableband pass filter'65. The circuit 66; including an amplifying tube A 67, is loosely coupled to -the inputcircuit of the detector D-l2to'supply thereto waves having thefrequency f f of the-intermediate frequency unmodulated carrier compo nent? These waves are combined with the side band waves transferred through'the filter 65 and'the 'ac'tionof the detector D12si-m-i larly as 'inythe' usual case is such as to repro du'cetherefrom' the speech currentsrepresented in saidside'bands, which sidebands' are based onthe combining intermediate carrier During transmission the outgoing signal-will be heard as side tone in receiver 13 and during'reception the incoming signal will be heard. A desired amount of thefumhodulated outgoing carrier waveha'ving frequency f as determined bythe ust ment of the suppressing means 45 and the circuit 68 maybe transmitted; It is assumed in the above amhe type of receiving circuit at theFdistant-station is such that only a very slight amount of unmodulated carrier wave shouldb'e' received, as 'would be the case, for example if itavere desired to prevent an intercep or fr'oiii reproducingthesignal by simple detec tions By elaboration of circuit and especiau ey means'of some type of double de't ectidr'i receiviiig'system, the desired receiver receive suc'lia wave and repro' duce the signalfrom it. The receiving cir cuit of'F 4 is not'dep'endent in any "way on theamdunter unmodula'ted carrier wave outgoing or incoming, since a separate local source, namely; circuit66 is used to supply the int'erinediat'e'frequency carrier Wave for thesec'ond stage of detection.

Whatis claimed is:

1. Ina modulating system, a thermionic modulator having an anode, cathode, and gr'idfan input circuit connected to said grid and cathode, and an output circuit connected to'sai'd' anode and catlidde, a source of waves ofcarrier and modulating frequency in said as to prevent the flow of unmodulated waves without regard to their'number or to their frquenciesthrough said output circuit, and means connected to saidoutput 'circuit'for freely transferring -modulated waves and Suppressingwaves df"frequencies ivhifihare 1 rnultiples of the frequency of the carrier wave source.

2. In a modulating system, an electron discharge device functioning as a madulator and having an anode, a cathode, and a grid, an input circuit connected to said grid and cathode, an output circuit connected to said cathode and anode, a source of carrier waves of relatively large amplitude and a source of modulating waves of relatively small amplitude connected to said input circuit, and an impedance element comprising only one type of impedance connected between said input and output circuits and adapted to prevent the flow of unmodulated waves through said output circuit.

3. In a modulatin system, an electronic modulator having an anode, a cathode and a grid, an input circuit connected to said grid and cathode, an output circuit connected to said anode and cathode, a source of waves of carrier frequency of relatively large amplitude and a source of waves of modulating frequency of relatively small amplitude connected to said input circuit, and an impedance network connected between said grid and anode having an impedance for waves of all frequencies including the modulating and carrier frequencies equal to the internal impedance of the electronic modulator divided by the amplification constant of the modulator.

4. In combination, a network and two circuits connected thereto, a space discharge device included in said network having anode, cathode, and discharge control ele ments, and a resistance element included in said network, so associated with said elements and of such constant value over a substantial frequency range as alone to prevent said network from transmitting waves of any frequency in said range from one of said circuits to the other.

5. A system comprising two circuits and a network connecting said circuits, a space discharg device included in said network, having anode, cathode, and discharge control elements, and a current path included in said network, connecting said anode and control elements and having an impedance element comprising only one type of impedance of such value for all frequencies of a substantial frequency range as to prevent said network from transmitting waves of any frequency in said range from one of said circuits to the other with greater transmission efficiency than any desired predetermined efficiency.

6. Means responsive to electric waves covering a band of frequencies for generating electric waves covering another frequency band, an output circuit for said means, means for supplying the first mentioned waves to said first mentioned means, and a circuit comprising substantially only resistance connected between said supply means and said output circuit and so electrically dimensioned relatively to the impedance of said output circuit as to prevent any of the first mentioned waves from appearing across said output circuit.

7. Means responsive to waves of different frequencies including a band of frequencies for generating waves of other frequencies, an output circuit connected to said means for utilizing certain of the latter waves, means to supply the first mentioned waves to said first mentioned means, and a circuit comprising substantially only resistance connected between said supply means and said output circuit and so electrically dimensioned relatively to the impedance of said output circuit as to prevent Waves of any of the first mentioned frequencies from flowing in said output circuit.

In witness whereof, I hereunto subscribe my name this 8th day of June, A. D. 1927.

JOHN F. FARRINGTON. 

